Field of the Invention
The invention relates to the use of localized electric field enhancement and, more particularly, to a device in which preformed triple junctions are provided in systems where insulating deposits form as part of normal operation.
Candidate conductors typically operate below the threshold for electrical breakdown in clean, ideal conditions. This allows for the degradation of voltage holding associated with the contamination expected in normal operation. Deposits can be a loose, unbound accumulation of material, such as, dust. Deposits can also take the form of an attached layer, or, be formed by chemical reaction. Examples where insulating deposits can form include corona rings, insulator gradient rings, and spark gaps exposed to contamination, such as, spark plugs. Electrodes in vacuum systems, such as, charged particle accelerators and plasma tools, are given special attention.
In liquid, gas, or vacuum environments, electrode contaminants can require maintenance, or, lead to high voltage breakdown. In semiconductor vacuum based manufacturing tools, such as, plasmas and ion beams, breakdowns can increase particle generation. On powered electrodes, hard power supply breakdowns (also called ‘glitches’), can significantly increase particle generation. This is undesirable, as particles can cause yield loss in semiconductor manufacturing and, consequently, particle counts are routinely monitored. Tool qualification and continued operation requires maintaining particle counts below a maximum allowable tolerance. What is needed is a technology that can increase service life by maintaining threshold particle counts, which would increase service life and be economically desirable.
Electrodes, Insulators, and Triple Junctions:
Insulators are critical components of any electrical system, and mechanisms that cause insulator failure have been extensively studied. In vacuum, gas, or non-conducting liquid systems, the confluence of medium-metal-insulator is called a “triple junction” (or “triple point”). Electric field enhancement at vacuum triple junctions can cause insulator breakdown. See, for example, Schächter, “Analytic expression for triple-point electron emission from an ideal edge”, Appl. Phys. Lett. 72 (4), 26 Jan. 1998; Chung et al., “Configuration-dependent enhancements of electric fields near the quadruple and the triple junction”, J. Vac. Sci. Tech. B28, C2A94, 2010; Stygar et al., “Improved design of a high-voltage vacuum-insulator interface”, Phys. Rev. ST Accel Beams 8, 050401 (2005).
In vacuum, an acute angle of intersection between insulator and cathode on the vacuum side can create a mathematical electric field singularity at the cathode triple junction. Then, electron field emission can create a creeping discharge along the insulator surface, and lead to eventual failure. An obtuse angle between insulator and anode can create a mathematical singularity in the electric field at the anode triple junction, which can lead to bulk insulator breakdown. Referring now to FIG. 1, one goal of research and development, according to U.S. Patent Application Publication No. 2014/0184055, has been to increase insulator service life by designing for an obtuse vacuum side angle 10 at the cathode 12 triple junction and an acute angle θ at the anode 14 triple junction. Concern for triple junction field enhanced breakdown extends from macro to submicron feature size. See, for example, U.S. Pat. No. 5,739,628 to Takada.
Study of vacuum triple junction breakdown is often done with clean vacuum, without free charge or ionizing radiation. So, study of the effect of deposits in a beam or plasma environments is relatively limited. However, even conductive deposits on electrodes in an ion beam can reduce the breakdown voltage to a fraction of that for clean electrodes. See, for example, Vanderberg, et. al., “Evaluation of electrode materials for ion implanters”, IEEE 0-7803-X/99, pp. 207-210.
The presence of plasma introduces multiple issues compared with clean vacuum. Plasma provides free charge (mostly electrons) and radiation, especially UV, which has sufficient energy for ionization. Even a diffuse plasma implies Debye shielding for features larger than 0.01 cm. Free charge and UV radiation are obviously detrimental to insulator integrity, but the effect of Debye shielding is less obvious. Plasma sheaths are dynamic, with extremely high fluctuating electric fields over small distances. More importantly, the conformal nature of the sheath effectively makes the time averaged sheath electric field orthogonal to the material boundary. For example, in the case of a positive plasma or beam potential, a grounded boundary is at cold cathode potential, with the plasma as anode. At a triple junction, this effectively reproduces the long discarded geometry of a triple junction insulator at 90 degrees to the anode.
Semiconductor plasma and beam systems can be dc, rf, and/or pulse powered. They are used for etching, cleaning, doping, and material deposition. Semiconductor processes can be particularly harsh, sometimes even including simultaneous refractory temperatures, oxidizing chemicals, and energetic particle bombardment. Electrodes can accumulate deposits as process by-products. Electrodes that function as cathodes or cold cathodes in ion beam, plasma and other electrical systems and devices can lose emissivity if the surface becomes insulating. The surface can become insulating if it is covered by insulating/dielectric deposits, or, if its materials are modified to have dielectric properties. Insulating deposits are particularly troublesome, especially in the presence of free charge or ionizing radiation. Formation of in situ triple junctions can alleviate this loss of emissivity because they introduce enhanced emissivity.
Many systems have an intrinsic form of directionality. For large scale, high voltage systems exposed to atmosphere, this may be simply be gravity. Plasma and ion beam processes frequently rely on energetic particles. In this case, deposit formation can have directionality imparted by either the energetic particles, or, by material backscattered from the impact of energetic particles.
Transient, low current electrical breakdown activity is routinely present in systems with high electric field. In air or vacuum, this is called corona. Corona cleaning, or, plasma discharge cleaning, is well known, and has often been used as a conditioning process for high voltage electrodes. Transient activity can be monitored by fast tracking of electrode current or voltage. The definition of ‘breakdown’ is subjective, depending on system requirements. Some systems simply run until complete insulator failure. In other cases, the onset of transient discharges that exceed a current or voltage threshold triggers a power supply interruption to limit damage.